iphone – iOS PJSIP: What will be the explanation of such error `PJSIP syntax error exception when parsing ‘Request Line’ header …`

I’ve a bizarre challenge with pjsip in my iOS venture.

The primary case:

• iPhone 11 with iOS 15.4
• Free WiFi community [free wi-fi endpoint like in airport/hotel]
• VPN is OFF
• UDP transport

[SIPSDK]: ERROR: sip_transport.c Error processing 527 bytes packet from UDP xx.xx.xxx.x:5060 : PJSIP syntax error exception when parsing 'Request Line' header on line 1 col 3:
SI�[2.0 401 Unauthorized

As I can see this message is broken SI�[2.0. Should be SIP/2.0. I will not able to establish connection to my pbx server.

The second case:

• the same iPhone 11 with iOS 15.4
• The same free WiFi network
• VPN is ON
• UDP transport

Everything is OK and the same message is not broken (I be able to establish connection to my pbx server).

SIP/2.0 401 Unauthorized

The third case:

• iPad Pro with iPadOS 15.4
• The same free WiFi network
• VPN is OFF
• UDP transport

Everything is OK and the same message is not broken.

My question is:

• what did break the sip message in the first case ?
• can it be some security attack thru free wi-fi endpoint (e.g. middle man) ?
• why do I see this problem only on iPhone device but iPad is working as usual?